Use of a random access memory input/output ("I/O") device for cache memory is well known in the art. Such devices are limited in their available array bandwidths by their use of a single I/O port. Some video memory devices provide a second (serial) port that is useful for transferring data serially into and out of the memory cells of the memory. However, this serial port is constrained to handle only sequences of data of length up to some predetermined number, such as 128 or 256, and from consecutive memory cells in a row or column of the cache. Further, the maximum number of cells that may be read out sequentially at a maximum rate from such a port is one row, such as 512 or 1,024 consecutive memory cells. These video memories are designed for video displays and are not particularly useful for other applications; they do not provide for internal byte counting or continuous transfers of indeterminate length. It would be useful if a serial port would allow transfer in and out of the contents of sequences of consecutive memory cells of smaller lengths, as low as 16 cells, and of larger lengths such as 2,048 cells.
The Texas Instrument TMS4461 multiport video RAM provides a random access port and a single serial port for data transfer into and out of a RAM memory, but the serial data word length appears to be limited to a single value, 256 consecutive words. Similarly, the NEC .mu.PD41264 memory provides a random access port and a single .times.4 serial port for data transfer, and serial data length is limited to the single value of 256, representing one fourth of the length of one row in a memory matrix. Because the output is .times.4, the length of one row is 256.times.4=1024.